1. Field of the Invention
The present invention relates to a secondary air supply device for supplying secondary air with the aid of pulsations in exhaust gases in order to promote further burning of HC, CO and the like in an exhaust gas system of an engine, which is particularly capable of preventing a noise in a secondary air passage upstream of a reed valve for controlling the secondary air flow and more particularly to a reed valve suitable for use in such a secondary air supply device.
2. Description of the Prior Art
In secondary air supply devices utilizing pulsations in exhaust gases, opening and closing operations of reed valves generally cause great noises. These noises are caused by (1) periodical movements of valve members every discharging of the exhaust gases from engine, (2) impacts and transmission thereof when the valve members sit on valve seats and (3) rapid changes in pressure of the secondary air resulting from the closing and opening of the valve members.
In order to prevent the noise it has been suggested to cause the secondary air to flow through an air cleaner for an engine so as to silence the noise in the air cleaner. Since, however, the air cleaner for the engine is generally remote from an exhaust tube, a secondary air passage is unavoidably elongated to increase its intake resistance and complicate its piping which necessarily make the device expensive.
With a conventional reed valve including a plurality of reed valve members, required numbers of component parts of the valve are prepared and respective valve members are arranged independently or in series in either event that the reed valve members are separately arranged or bodily united. Such a reed valve becomes bulky and heavy and expensive to manufacture due to an increase of the number of parts with complicated secondary air supply piping and further makes a noise when the valve members are opened and closed.
In order to solve this problem, it has been proposed to arrange the valve members radially. With this arrangement, as the valve member is centrally fixed by means of only one set screw, the membraneous valve member is likely to deform at its bottom resulting in an incomplete contact between the valve member and a valve seat. The valve member is apt to rotate when being subjected to vibrations, with the result that upstream and downstream sides of the valve seat are always communicated with each other. In addition to these problems in durability, a stopper is of a circular thick configuration to increase its weight and its continuous wide curved surface increases the flow resistance because the secondary air which has passed through the valve member would impinges against the wide curved surface of the stopper.
For the purpose of solving these problems it has been considered to make large a reed valve member. A large-sized reed valve member in the form of a cantilever, however, has a low resonance frequency (for example, 140 Hz) which adversely affects its durability in use and air flow characteristics. Moreover, when a great quantity of air flow passes through a reed valve having a single valve member, its lift becomes longer to cause impacts between the valve member and its valve seat giving rise to the problem of noise. In view of these facts, with a single reed valve member it is disadvantageous to aim to obtain a great quantity of secondary air flow.